Transferrin in FRTL5 cells: regulation of its receptor by mitogenic agents and its role in growth.

Transferrin, a serum iron-binding protein, delivers iron to the cell after binding to specific receptors on the cell surface and is an important component of culture medium for virtually all cell lines, including the FRTL5 line of rat thyroid follicular cells. Therefore, we undertook studies in FRTL5 cells to examine the regulation of the transferrin receptor, the effects of transferrin on growth and differentiated functions, and the interactions of transferrin with several mitogenic pathways. FRTL5 cells possess one class of saturable transferrin receptors (Ka, 0.7 x 10(9) M-1). Binding of 125I-labeled transferrin was highest in actively growing cells and declined progressively, reaching minimal values when confluence was achieved. Removal of transferrin from culture medium caused a rapid increase in transferrin binding. TSH, acting within 5 min, induced a modest increase in transferrin binding, due to a cycloheximide-resistant increase in binding sites. Binding of transferrin after a 24-h incubation was also increased by other mitogenic agents, (Bu)2cAMP, forskolin (FK), insulin, insulin-like growth factor-I (IGF-I), and the phorbol ester TPA. Transferrin alone stimulated growth only minimally, but enhanced the mitogenic effect of TSH, (Bu)2cAMP, and FK, all of which act through the cAMP pathway. In contrast, transferrin did not alter the cAMP-independent mitogenic effects of insulin and IGF-I. Transferrin did not affect TSH-induced cAMP generation. Desferoxamine, an iron chelator, inhibited the mitogenic effects of all of the agents tested. Desferoxamine had no significant effect on TSH-induced cAMP accumulation. We conclude that FRTL5 cells contain saturable receptors for transferrin whose abundance varies with the rate of cell replication. Transferrin down-regulates its own receptors, while stimulation of growth by various mitogens is accompanied by increased binding of transferrin. Transferrin enhances the mitogenic effect of the cAMP-dependent mitogens, TSH, (Bu)2cAMP, and FK, without modifying basal or stimulated cAMP generation. In contrast, transferrin fails to affect the mitogenic responses to IGF-I and insulin, which are cAMP independent. Iron is required for the mitogenic response to various mitogens, especially those that are cAMP dependent.